"Ian G" wrote in message
...
18mm ply glued and screwed to the top of the joists (instead of unglued
chipboard) , forming a T beam was a solution proposed by a structural
engineer and adopted by me when I did a loft conversion some years ago. We
didn't put a piano up there, but the purchasers of the house did install a
water bed and there have been no adverse reports from neighbours whom I
have remained in contact with. The increase in strength over the unglued
floor was impressive, confirming in practice what the Moment of Inertia
calculations indicated. We were able to jump up and down on the floor and
detect very little spring.

I would expect it to be even better if you fix a steel plate to the bottom
and ply to the top.
Steel is better in tension.
Just fixing a 3mm steel plate to either side of the joists will make a huge
difference as the wood stops the plate from warping and the 4" depth of
steel doesn't bend much. My conservatory roof is supported in a similar way
by ally strips inserted in channels in the plastic and I can walk on it and
I am not thin.